Orthotic devices serve as essential elements of conservative care programs for posterior tibial tendon dysfunction (PTTD). Data on currently used devices remain limited. Current device designs are restrictive and focus on correction of flatfoot kinematics in subjects with stage II PTTD. Restrictive designs that limit ankle motion may lead to weakness and altered gait dynamics. It remains unclear if less restrictive device designs would be more optimal for subjects with stage II PTTD. The aims of the current proposal are to examine the in- vivo biomechanics of different device designs and to use outcomes to identify the device and design optimal for subjects with stage II PTTD. Research Design: This study will use a laboratory based repeated measures design to explore the function of four different orthotic devices in a sample of 60 subjects with stage II PTTD at baseline. Then each subject will be followed prospectively for 12 weeks while wearing one of the orthotic devices. Clinical outcomes including strength, tendon morphology, and self-reported outcomes will be measured at 12 weeks. Baseline foot kinematics will be collected using a three segment foot model (first metatarsal, calcaneus, tibia) while data on ankle kinetics will be derived from the kinematic data and the ground reaction forces. Impact: There are currently no head-to-head comparisons available among the various devices used in the conservative management of stage II PTTD. The proposed study will provide the necessary controlled data to determine the kinematic and kinetic effects of selected devices and the specific components needed to optimize device design. Outcomes will be used to determine which designs are successful to use clinically. PUBLIC HEALTH RELEVANCE: The lack of high quality evidence to guide conservative care for patients with posterior tibial tendon dysfunction (PTTD) has led to controversy over clinical care. Numerous orthotic devices are available to avoid a costly and debilitating surgery but no consensus on which to use has been made. The current study aims to link biomechanical testing of selected devices to clinical outcomes in subjects with PTTD after wearing a device for 12 weeks.